Activator for Pulp Bleaching

ABSTRACT

A process for the bleaching of cellulosic fibres comprising the steps of (a) providing a water supply; (b) adding hydrogen peroxide to the stabilised water supply to form a hydrogen peroxide solution; (c) adjusting the pH of the hydrogen peroxide solution by adding alkali; mixing the pH adjusted solution resulting from step (c) to allow complete formation of perhydroxyl ion; (e) adding a carboxylic acid anhydride compound to the solution resulting from step (d); (f) mixing the solution resulting from step (e) to allow complete formation of percarboxylic acid to give a dilute solution of percarboxylic acid; and (g) contacting the solution with a suspension of cellulosic fibres.

This invention relates to a process for generating a dilute solution ofpercarboxylic acid which is subsequently used to bleach wood or non woodpulp. More specifically, in this process the bleaching solution isformed by reaction of hydrogen peroxide with a carboxylic acid anhydridecompound which behaves as a bleach activator.

Wood and non wood pulp are valuable raw materials in the paper industryand it is desirable that a high degree of whiteness is obtainable.

Oxygen based bleaching is used for pulp bleaching because of itsenvironmental benefits and oxidising power. Hydrogen peroxide isincreasingly being used in oxygen based pulp bleaching.

Hydrogen peroxide, however, suffers from some performance disadvantageswhich become particularly important when an existing pulp bleachingsequence is being converted to operate with hydrogen peroxide. One majorproblem is the fact that the degree of whiteness may be reduced comparedwith the use of chlorine based bleaches which significantly reduces thevalue of the pulp.

In order to overcome these problems work has gone into developing analternative bleaching solution. It is proposed in various patentapplications to react at least part of the hydrogen peroxide with ableach activator thereby generating a peroxyacid based oxygen bleachingspecies in situ. Bleach activators contain a good leaving group suchthat on mixing with hydrogen peroxide, reaction takes place to giveperacid species which are more effective bleaching agents than hydrogenperoxide itself.

EP-A-0670929 describes a method for bleaching lignocellulose containingpulp in which equilibrium peracid is generated by reacting acetic acidand hydrogen peroxide. It also suggests that in situ peracid can begenerated by reaction of acetic anhydride with hydrogen peroxidedirectly in the bleaching stage although the equilibrium peracid ispreferred and is what is used in the examples of this application.EP-A-0670928 describes an identical method. However, whilst equilibriumperacid improves the bleaching it also introduces problems of handlingand dosing, peracid in the pulp plant.

Further examples such as EP456032 describe the use oftetraacetylethylenediamine (TAED) as the bleach activator. Whilst thisproduces a very effective bleaching solution, generation of such asolution in certain circumstances may not prove to be economical for allmills. It would therefore be desirable to develop a process by whichperacid can be generated in a good yield by reacting hydrogen peroxidewith a more cost effective bleach activator, thus ensuring that it ismore cost effective than the method of the prior art which uses TAED. Ableach activator suitable for solving this problem is acetic anhydride.Whilst the use of acetic anhydride as a bleach activator to produceperacetic acid is well documented, surprisingly better performance canbe delivered using peracetic acid generated from acetic anhydride ascompared to preformed peracetic acid. Furthermore there is yet to bedeveloped a simple and inexpensive process by which good yields ofperacid can be produced.

According to the present invention there is provided a process for thebleaching of cellulosic fibres comprising the steps of;

-   a) treating a fresh water supply with a chelating agent to give a    stabilised water supply;-   b) adding hydrogen peroxide to the stabilised water supply to form a    hydrogen peroxide solution;-   c) adjusting the pH of the hydrogen peroxide solution by adding    alkali;-   d) mixing the pH adjusted solution resulting from step (c) to allow    complete formation of perhydroxyl ion;-   e) adding a carboxylic acid anhydride compound to the solution    resulting from step (d);-   f) mixing the solution resulting from step (e) to allow complete    formation of percarboxylic acid to give a dilute solution of    percarboxylic acid; and-   g) contacting the percarboxylic acid solution with a suspension of    cellulosic fibres.

It is preferable that the water used in step a) is of a temperaturebetween 5° C.-100° C., preferably between 10° C.-50° C. and morepreferably between 15° C.-30° C. At cooler temperatures the generationof the percarboxylic acid may be inhibited whilst at highertemperatures, although the generation would be rapid, it is anticipatedthat degradation of the percarboxylic acid species would be promoted.

Chelating agent is added to the water in step a) prior to the additionof further components and is advantageous as it ensures that any tracemetals which may be present in the supply are removed. Such trace metalscould potentially promote the decomposition of hydrogen peroxide. Amixing step is preferably included to maximise the chelation of thetrace metals.

Preferably the chelating agent is an aqueous phosphonic acid basedchelating agent. Examples of suitable phosphonic acid based chelatingagents are polyamino methylene phosphonic acids such as based onethylene diamine or diethylenetriamine. Commercially available compoundsof this type include Dequest SPE9505, Dequest 20605. Dequest 2066,Dequest 2066A manufactured by Solutia and Versenate PS manufactured byDOW. In a preferred embodiment the chelating agent is Dequest SPE 9505and/or Dequest 2066. An aqueous phosphonic acid based chelating agenthas been found to work best under the conditions of the method of thepresent invention.

Preferably the amount of chelating agent, eg phosphonic acid basedagent, added to the water is 0.1-0.4 wt %, more preferably 0.125-0.3% wtand most preferably 0.15-0.25% wt.

Preferably the amount of hydrogen peroxide added in step b), based on a50% solution, is a value in the range 0.5-6 wt %, more preferably in therange 1.5-4.0 wt % and most preferably in the range 1.75-2.5 wt %.

It is desirable that the addition of hydrogen peroxide is followed by anin line mixing step. This mixing step is often required because thewater and hydrogen peroxide are transferred separately from theirholding containers to the vessel in which the reaction proceeds. It istherefore beneficial to include a mixing step to ensure that a uniformsolution is obtained.

The alkali is added in step c) in an amount to raise the pH of thesolution from a value in the range 6-7 to a value in the range10.5-11.5. It is important that this alkali addition occurs prior tostep (d). This is because rendering the hydrogen peroxide solution ofstep b) alkaline promotes the formation of perhydroxyl ion with whichthe carboxylic acid anhydride compound preferentially reacts instead ofwater. If step c) was not included, the carboxylic acid anhydridecompound would hydrolyse thus reducing the amount of percarboxylic acideventually formed. Suitable alkalis include potassium hydroxide, calciumhydroxide, magnesium hydroxide and sodium hydroxide. Preferably thealkali is sodium hydroxide. It is important to raise the pH of thesolution because it is at alkaline pHs that formation of the perhydroxylion is promoted. Sodium hydroxide is available as a 30 w/w % or 50 w/w %solution. Suitable amounts were determined by monitoring the pH.Preferably the amount of sodium hydroxide added based on a 50% solutionis a value in the range 0.2-3 wt %, more preferably in the range 0.4-1.5wt % and most preferably in the range 0.5-1 wt %.

Following the addition of the alkali, there is a further mixing step(step d)). The duration of this step is preferably at least one minute.This is important to optimise the extent of formation of the perhydroxylion in the subsequent steps. At present, a specific method for directlyanalysing whether complete formation of the perhydroxyl ion has takenplace is not available. However, it is possible to obtain an indicationof the extent of perhydroxyl ion formation by analysing the percentageconversion of the carboxylic acid anhydride compound to percarboxylicacid. A low percentage conversion rate may indicate incomplete formationof the perhydroxyl ion. If sufficient time is not allowed for thismixing step and the carboxylic acid anhydride compound is addedprematurely then it would preferentially undergo a hydrolysis reactionwith water resulting in a low percarboxylic acid generation.

After sufficient mixing in step d) has taken place, the carboxylic acidanhydride compound is added (step e)). Preferably the carboxylic acidanhydride compound added in step e) is selected from one or more of thegroup consisting of acetic anhydride, maleic anhydride, succinicanhydride, phthalic anhydride, malonic anhydride, benzoic anhydride andpropanoic anhydride. Preferably the amount of carboxylic acid anhydridecompound added is in the range 0.02-0.6 mol %, more preferably in therange 0.05-0.3 mol % and most preferably in the range 0.075-0.15 mol %and this is addition is followed by a further important mixing step(step f)). This mixing step should be allowed to occur for at least 10minutes. This is to maximise generation of the percarboxylic acidspecies. It is, however, possible to accelerate the generation of thepercarboxylic acid species by using amounts of alkali at the higher endof the ranges mentioned above prior to the carboxylic acid anhydrideaddition.

Preferably the carboxylic acid anhydride compound comprises aceticanhydride. Acetic anhydride has a flash point of 54° C. and as such isclassified as flammable. A compound is classified as flammable where ithas a flashpoint in the range of 21° C.-55° C. This means that themodification of old mill equipment, although minor, will requireconsideration of the Dangerous Substances and Explosive AtmosphereRegulations (DSEAR). Flammability rating also has an impact on zoneclassification on manufacturing plants. In order to further minimise theminor modifications required, the applicants have investigatedincreasing the flash point of acetic anhydride by mixing with a furthercarboxylic acid anhydride with a higher flashpoint. For example, maleicanhydride has a flash point of 102° C. Samples of 80:20 and 50:50 aceticanhydride:maleic anhydride solutions were prepared and it was found thatin both cases the flash point was increased, to 60° C. and 65° C.respectively, meaning that the solutions would not be classified asflammable. Further testing also surprisingly showed that these solutionswere as effective as a bleach activator as a solution of aceticanhydride only.

Thus in one embodiment of the present invention, the flashpoint ofacetic anhydride is modified by addition of a carboxylic acid anhydridewith a higher flashpoint to form a mixture which has a flash point of55.1° C. or higher such that it is not classified as flammable.Preferably a mixture which has a flash point of 60° C. or higher isformed. Preferably the amount of carboxylic acid anhydride with a higherflashpoint added is such that the resulting mixture has a ratio ofacetic anhydride to carboxylic acid anhydride with a higher flashpointgreater than 50:50 but less than or equal to 80:20. Preferably thecarboxylic acid anhydride with a higher flashpoint is maleic anhydride.A 50:50 mixture of acetic anhydride:maleic anhydride is not commerciallypractical as, upon cooling, the maleic anhydride precipitates out of thesolution. Preferably the ratio of acetic anydride to maleic anhydride is80:20. Preferably the mixture consists of the acetic anhydride andfurther carboxylic acid anhydride with a higher flashpoint only.

The mixture of acetic anhydride and carboxylic acid anhydride with ahigher flashpoint can then be used as the carboxylic acid anhydridecompound in the method for bleaching cellulosic fibres of the invention.

It is anticipated however, that the mixture of acetic anhydride andcarboxylic acid anhydride with a higher flash point may be useful invarious applications not just limited to the bleaching of cellulosicfibres. The method of increasing the flash point of acetic anhydride bymixing with a carboxylic acid anhydride with a higher flashpoint and themixture itself form a second aspect of the invention.

The molar ratio of carboxylic acid anhydride compound to hydrogenperoxide added is preferably in the range 1:1 to 1:10, preferably 1:1.5to 1:6, more preferably 1:2 to 1:4. Most preferably the molar ratio isabout 1:3. In terms of the number of moles added, it is preferable thatmore hydrogen peroxide is required than carboxylic acid anhydridecompound in order to drive the reaction to form perhydroxyl ion.

The resultant dilute percarboxylic acid solution formed in step f) willhave a pH of approximately 5. Depending on the mill conditions, it maybe necessary to increase the solution pH to suit the bleachingconditions required in the final step of the process, step g).

Typically a conversion of carboxylic acid anhydride compound topercarboxylic acid of 90-95% is obtained. Preferably the resultantsolution contains percarboxylic acid in a concentration range of0.1-10%, more preferably 0.25-5% and most preferably 0.5-2%.

The final step (step g)) of the process is to contact the dilutepercarboxylic acid solution with a cellulosic fibre suspension e.gformed from pulp. The pulp may be any sort of pulp, including chemicaland mechanical pulp and mixture thereof, including recycled material.Wood and non wood fibres can be bleached using this process. The productmay be used directly to form paper or board or may be fully or partiallydewatered to form a pulp intermediate for eventual paper or boardmanufacture.

The following examples give an indication of how effective this processis and demonstrate the high yield of percarboxylic acid which can beexpected to be generated.

DESCRIPTION OF THE FIGURES

FIG. 1 shows the percarboxylic acid release profile of formulations 1-5as detailed in Example 1;

FIG. 2 shows the pH profile of formulations 1-5 as detailed in Example1;

FIG. 3 shows the brownstock delignification of soft wood pulp using a0.5% peracetic acid charge from anhydride sources formulations 1-5 interms of Kappa as detailed in example 2;

FIG. 4 shows the brownstock delignification of soft wood pulp using a0.5% peracetic acid charge from anhydride source formulations 1-5 interms of ISO brightness as detailed in Example 2;

FIG. 5 shows the storage chest bleaching of fully bleached pulp usingperacetic acid from formulations 1-5 measured in terms of ISO brightnessas detailed in Example 3; and

FIG. 6 shows the recycled fibre (80:20 Newsprint:MOW) bleaching usingperacetic acid from formulations 1-5 measured in terms of ISO Brightnessas detailed in Example 4.

FIG. 7 shows a comparison of preformed and acetic anhydride derivedperacetic acid on bleaching brownstock pulp.

FIG. 8 shows a comparison of preformed and acetic anhydride derivedperacetic acid on bleaching 80:20 Newsprint:MOW recycled furnish.

EXAMPLES

In order to compare the bleaching ability of the acetic anhydridederived peracetic acid solutions with those derived from TAED as arecommonly used in the art, various formulations were prepared. It wasalso the case that formulations containing an acetic anhydride/maleicanhydride solution were tested.

The formulations prepared are as detailed below: FORMULATION 1:FORMULATION 2: Water 962 g Water 962 g Dequest 2066 2 g Dequest 2 g H₂O₂(50%) 21 g H₂O₂ (50%) 21 g NaOH (50%) 7.5 g NaOH (50%) 7.5 g 80:20 AC:Mal 10.7 g 50:50 AC: Mal 10.5 g Anhydride Anhydride

FORMULATION 3: FORMULATION 4: Water 962 g Water 962 g Dequest 2066 2 gDequest 2066 2 g H₂O₂ (50%) 21 g H₂O₂ (50%) 21 g NaOH (50%) 7.5 g NaOH(50%) 7.5 g Maleic Anhydride 10.3 g Acetic Anhydride 1O.7 g

FORMULATION 5: Water 962 g Dequest 2066 2 g H₂O₂ (50%) 21 g P420 15 gNaOH 5.5 gAC: Mal=Acetic anhydride: Maleic AnhydrideP420=TAED based activator (PeroxyBoost (RTM))

Each of the formulations were prepared according to the method of thepresent invention. More specifically, fresh water at a temperature inthe range from 15-30° C. was treated with 0.2 wt % of Dequest 2066. 2 wt% of a 50% hydrogen peroxide solution was added to the stabilised watersupply. the pH of the resulting hydrogen peroxide solution was adjustedto a pH in the range from 10.5 to 11.5 by the addition 0.75 wt % of a50% solution of sodium hydroxide. The mixture was then mixed for aperiod of 1 minute to allow complete formation of the perhydroxyl ion.The bleach activator was added to this mixed solution in the amountsshown above respectively and the solution was mixed to allow completeformation of peracetic acid to give a dilute solution of peracetic acid.

Example 1

Peracid Release/pH Profile

The peracid release profile for each of formulations 1-5 was determinedby taking measurements between 2 and 145 minutes. The solutiontemperatures were maintained at 25° C. pH profiles were also attainedduring the process. The results of the peracid release profile are shownin Table 1 and FIG. 1. The pH profiles attained are illustrated in Table2 and FIG. 2. Time mins Formulation 1 Formulation 2 Formulation 3Formulation 4 Formulation 5 % Peracid release 2 89.6 85 65.2 % Peracidrelease 3 85 % Peracid release 4 80 % Peracid release 5 86.6 % Peracidrelease 6 93.8 % Peracid release 7 88.6 % Peracid release 10 91.8 91.489.6 % Peracid release 11 89.6 % Peracid release 13 95.3 % Peracidrelease 15 91.8 88.1 % Peracid release 16 93.8 % Peracid release 20 9089 93.3 87.6 % Peracid release 21 87.6 % Peracid release 25 87.6 93.884.6 % Peracid release 30 88.1 84.3 85 93.8 83.6 % Peracid release 4584.8 80 % Peracid release 60 82.5 82.3 92.9 % Peracid release 120 75.8 %Peracid release 145 74

TABLE 2 Formulation 1 Formulation 2 Formulation 3 Formulation 4Formulation 5 80:20 acetic:maleic 50:50 acetic:maleic 100% maleic 100%acetic PeroxyBoost ® anhydride mix anhydride mix anhydride anhydrideP420 Time (mins) pH pH pH pH pH 0 11.22 11.2 11.24 11.22 10.89 0.5 7.117 10 5.3 10.44 1 5.03 4.93 8.01 5.3 9.9 2 5 4.84 7.5 5.28 9.07 3 6.62 44.99 4.82 5.44 5.25 8.69 6 3.75 8.59 8 4.97 4.8 3.38 5.24 8.46 10 4.964.8 3.3 5.24 8.4

Example 2

Brownstock Delignification

Using formulations 1-5, brownstock delignification was carried out. Theexperimental conditions and results are summarised in Table 3. In orderto determine how effectively the different formulations worked usingsoftwood brownstock pulp, kappa and ISO brightness data was recordedusing International Standards, ISO 302-1981 (E) and ISO 3688-1977respectively. The results are also illustrated graphically in FIGS. 3and 4. TABLE 3 Stage Conditions/Results Formulation 1 Formulation 2Formulation 3 Formulation 4 Formulation 5 Brownstock-P_(A) PeracidCharge 0.5% 0.5% 0.5% 0.5% 0.5% Caustic charge 0.36% 0.38% 0.39% 0.35%0.40% Temperature 65° C. 65° C. 65° C. 65° C. 65° C. Initial pH 8.038.48 8.50 8.32 8.10 Final pH 6.21 5.66 6.51 6.11 5.51 Consistency 10% 10%  10%  10%  10%  Retention time 120 min 120 min 120 min 120 min 120min Pre stage brightness 33.8 33.8 33.8 33.8 33.8 Post stage brightness46.4% 44.2% 44.7% 44.9% 46.3% Pre stage Kappa 18.9 18.9 18.9 18.9 18.9Post stage Kappa 10.4 11.1 10.2 10.4 10.5 Storage chest-P_(A) peracidCharge 0.5% 0.5% 0.5% 0.5% 0.5% Caustic charge 0.10% 0.12% 0.13% 0.14%0.16% Temperature 65° C. 65° C. 65° C. 65° C. 65° C. Initial pH 8.238.23 8.29 8.38 8.29 Final pH 5.77 7.05 7.08 7.23 6.23 Consistency 10% 10%  10%  10%  10%  Retention time 120 min 120 min 120 min 120 min 120min Pre stage brightness 85.9% 85.9% 85.9% 85.9% 85.9% Post stagebrightness 86.8% 86.8% 87.0% 87.1% 87.2% Recycled fibre-P_(A) (80:20newsprint:MOW) Peracid Charge 0.5% 0.5% 0.5% 0.5% 0.5% Caustic charge0.19% 0.14% 0.20% 0.19% 0.14% Temperature 65° C. 65° C. 65° C. 65° C.65° C. Initial pH 8.13 7.85 8.08 8.12 8.0 Final pH 7.13 7.05 7.07 7.237.36 Consistency 10%  10%  10%  10%  10%  Retention time 120 min 120 min120 min 120 min 120 min Pre stage brightness 83.2% 83.2% 83.2% 83.2%83.2% Post stage brightness 87.8% 87.2% 87.2% 88.1% 87.7%

Example 3

Storage Chest Bleaching

Using formulations 1-5, a storage chest bleaching study was performedusing a fully bleached pulp. Kappa and ISO brightness data was recordedusing International Standards, ISO 302-1981 (E) and ISO 3688-1977respectively. The experimental conditions and results are summarised inTable 3. The results obtained are also illustrated graphically in FIG.5.

Example 4

Recycled Fibre Bleaching Study

Using formulations 1-5, a recycled fibre bleaching study was carried outusing 80:20 Newsprint:MOW grade furnish. ISO brightness data wasrecorded using International Standards, ISO 302-1981(E) and ISO3688-1977 respectively. The experimental conditions and results aresummarised in Table 3. The results obtained are also illustratedgraphically in FIG. 6.

Conclusions

The results obtained from the experiments performed clearly demonstratethat performance similar to that achieved using a PeroxyBoost (RTM) TAEDbleach activator can be obtained by using acetic anhydride as requiredby the present invention. It has further been shown that adding maleicanhydride to the acetic anhydride to form a stabilised aceticanhydride/maleic anhydride solution does not affect the peracid release.

Furthermore, the bleaching studies demonstrated that bleaching withperacetic acid derived from acetic anhydride provides similar results.The results when using a mixture of peracetic acid and permaleic acid orjust permaleic acid alone are also similar. Therefore, surprisingly, theintroduction of permaleic acid does not affect the bleaching ability ofperacetic acid.

In a set of further examples, the bleaching ability of the aceticanhydride derived peracetic acid solution was compared with thebleaching ability of a preformed peracetic acid. More specifically,bleaching formulation 4 was compared to a preformed commerciallyavailable peracetic acid (supplied by Aldrich), hereinafter referred toas formulation 6.

Using the peracetic acid solutions, brown stock delignification andrecycled bleaching studies were performed using brown stock and 80:20Newsprint:MOW pulps respectively. The studies were performed using a0.1%, 0.25% and 0.5% charge of peracetic acid (based on 100% peraceticacid) respectively at a temperature of 70° C. and with a retention timeof 120 minutes. Formulation 4 was used at a pH in the range 8-8.5 andformulation 6 was used at a pH of 5 which represents the normalconditions under which it is used.

ISO brightness data was recorded using International Standard ISO3688-1977. The results are shown graphically in FIGS. 7 and 8.

The data shows that a superior bleaching performance is achieved usingacetic anhydride derived peracetic acid as compared to preformedperacetic acid.

Flash Point Analysis

The flashpoints of various neat carboxylic acid anhydride compounds areillustrated in Table 4.

Samples of acetic anhydride, maleic anhydride, 80:20 aceticanhydride:maleic anhydride and 50:50 acetic anhydride:maleic anhydridewere prepared. The flash points of these solutions were determined usingthe Pensky Martens closed cup test. The results obtained are summarisedin Table 5. TABLE 4 Carboxylic acid anhydride Flashpoint (° C.) Aceticanhydride  54 Benzoic anhydride 110 Maleic anhydride 102 Phthalicanhydride 152 Propanoic anhydride  63 Succinic anhydride 157

TABLE 5 Bleach Activator Flash Point ° C. Acetic anhydride  54° C.Maleic anhydride 102° C. 80:20 acetic:maleic anhydride mix  60° C. 50:50acetic:maleic anhydride mix  65° C.Freezing Point Assessment

The freezing point of acetic and maleic anhydride mixtures was assessed.Acetic: maleic anhydride solutions with a total weight of 100 g, in theratios 80:20, 50:50 and 20:80 respectively were prepared. The sampleswere contained in a lidded glass storage jar and then stored for 2 daysat 17° C. The results are detailed in Table 5. TABLE 6 Physical stateafter storage at Acetic:maleic anhydride mixture −17° C. for 2-3 days80:20 acetic:maleic anhydride Liquid (no crystallisation) 50:50acetic:maleic anhydride Part solid / part liquid 20:80 acetic:maleicanhydride solid

This assessment showed that in order to be suitable, from a commercialpoint of view, as a liquid activator, a ratio of acetic:maleic anhydrideof greater than 50:50 is required.

1. A process for the bleaching of cellulosic fibres comprising the stepsof: (a) treating a fresh water supply with chelating agent to give astabilised water supply; (b) adding hydrogen peroxide to the stabilisedwater supply to form a hydrogen peroxide solution; (c) adjusting the pHof the hydrogen peroxide solution by adding alkali; (d) mixing the pHadjusted solution resulting from step (C) to allow complete formation ofperhydroxyl ion; (e) adding a carboxylic acid anhydride compound to thesolution resulting from step (d); (i) mixing the solution resulting fromstep (e) to allow complete formation of percarboxylic acid to give adilute solution of percarboxylic acid; and (g) contacting the solutionof peracid with a suspension of cellulosic fibres.
 2. The processaccording to claim 1 in which the fresh water supply is at a temperaturein the range 15-30° C.
 3. The process according to claim 1, wherein thechelating agent is an aqueous phosphonic acid based.
 4. The processaccording to claim 3, wherein the phosphonic acid based chelating agentis Dequest SPE9505 and/or Dequest
 2066. 5. The process according toclaim 1, wherein the amount of chelating agent added is in the range0.1-0.4 wt %.
 6. The process according to claim 1, wherein the amount ofchelating agent added is in the range from 0.125-0.3 wt %.
 7. Theprocess according to claim 1, wherein the amount of chelating agentadded is in the range 0.15-0.25 wt %
 8. A process according to claim 1wherein the amount of hydrogen peroxide added based on a 50% solution isin the range 0.5-6 wt %, more preferably in the range 1.5-4.0 wt % andmost preferably in the range 1.75-2.5 wt %.
 9. The process according toclaim 1 wherein the alkali is sodium hydroxide.
 10. A process accordingto claim 1 wherein the amount of alkali added is in the range 0.2-3 wt%, preferably in the range 0.4-1.5 wt % and more preferably in the range0.5-1.0 wt %.
 11. A process according to claim 1 wherein the pH isadjusted to a value in the range 10.5-11.5.
 12. A process according toclaim 1 in which the duration of the mixing step (d) is at least 1minute.
 13. A process according to claim 1 in which the amount ofcarboxylic acid anhydride compound added is in the range 0.02-0.6 mol %,preferably in the range 0.05-0.3 mol % and more preferably in the range0.075-0.15 mol %.
 14. A process according to claim 1 in which theduration of the mixing step (f) is up to 10 minutes.
 15. The processaccording to claim 1 which further comprises an additional pH adjustingstep prior to the solution being contacted with the fibrous cellulosicsolution.
 16. The process according to claim 1 wherein the carboxylicacid anhydride is selected from one or more of the group consisting ofacetic anhydride, maleic anhydride, succinic anhydride, malonicanhydride, benzoic anhydride and propanoic anhydride.
 17. The processaccording to claim 16, wherein the carboxylic acid anhydride compoundcomprises acetic anhydride.
 18. The process of claim 17, wherein thecarboxylic anhydride compound additionally comprises a furthercarboxylic acid anhydride with a higher flashpoint and the aceticanhydride has been premixed with the further carboxylic acid anhydridewith a higher flashpoint to form an acetic anhydride/carboxylic acidanhydride mixture with a modified flash point of 55.1° C. or above. 19.The process of claim 18 wherein the ratio of acetic anhydride to maleicanhydride in the acetic anhydride/carboxylic acid anhydride mixture witha modified flashpoint is greater than 50:50.
 20. The process of claim 18wherein the ratio of acetic to maleic anhydride with a higher flashpointin the acetic anhydride/carboxylic acid anhydride mixture with amodified flashpoint is 80:20.
 21. The process according to claim 19,wherein the carboxylic acid anhydride with a higher flashpoint is maleicanhydride.
 22. A method for increasing the flash point of aceticanhydride to at least 55.1° C. comprising forming a mixture of aceticanhydride and a further carboxylic acid anhydride with a higherflashpoint in order to form a acetic anhydride/carboxylic acid anhydridemixture with a modified flashpoint.
 23. The method of claim 22, whereinthe acetic anhydride/carboxylic acid anhydride mixture with a modifiedflashpoint has a ratio of acetic anhydride to carboxylic acid anhydridewith a higher flashpoint greater than 50:50.
 24. The method according toclaim 22, wherein the acetic/carboxylic acid anhydride mixture with amodified flashpoint has a ratio of acetic anhydride to carboxylic acidanhydride with a higher flashpoint of 80:20.
 25. The method according toclaim 22, wherein the carboxylic acid anhydride with a higher flashpointis maleic anhydride.
 26. A composition comprising acetic anhydride and afurther carboxylic acid anhydride with a higher flashpoint, which has aflashpoint of at least 55.1° C. and wherein the ratio of aceticanhydride to carboxylic acid anhydride with a higher flashpoint isgreater than 50:50 but less than or equal to 80:20.
 27. The compositionaccording to claim 26 which consists of acetic anhydride and a furthercarboxylic acid anhydride only.